Electro-optic modulators change refractive index of a branch of a waveguide (hereinafter the modulated branch) by an electro-optic effect of a modulating electric field. Thus, the electro-optic modulator can alter a phase of light waves traversing the modulated branch. As a result, a phase shift is generated between the light waves traversing the modulated branch and light waves traversing another branch of the waveguide (hereinafter the non-modulated branch). The light waves traversing the modulated branch interface with light waves traversing the non-modulated branch. Output energy of the light waves of the waveguide is modulated as the output energy depends on the phase shift, which in turn depends on the modulating electric field.
In theory, when an input energy Qin of light waves of the waveguide is about Q, and the output energy Qout of light waves of the waveguide is about Q, it represents that the electro-optic modulator is full open. When an input energy Qin of light waves of the waveguide is about Q, and the output energy Qout of light waves of the waveguide is about zero, it represents that the electro-optic modulator is full closed. A ratio between a maximum output energy Qout and a minimum output energy Qout is defined as an information ratio. The larger the information ratio is, the more apparent the electro-optic modulator is open or closed.